1. Field of the Invention
The present invention relates to an image forming apparatus in which a toner image on an image bearing member is transferred onto an intermediate transfer member, and then the toner image on the intermediate transfer member onto a transfer material.
2. Related Background Art
FIG. 5 is a schematic constructural view of a conventional image forming apparatus.
A photosensitive drum 101 formed from OPC (organic semi-conductor) is rotated in a direction shown by the arrow and a surface of the photosensitive drum is uniformly charged by a charger 102. Then, exposure corresponding to image information is effected by using an exposure device 103 having a light source 103a and a reflection mirror 103b, thereby forming an electrostatic latent image. Developing devices 104a, 104b, 104c and 104d containing yellow toner, magenta toner, cyan toner and black toner, respectively are mounted on a rotary 104A. By rotating the rotary 104A, the developing device (for example, yellow developing device 104a) to be used for development of the electrostatic latent image on the photosensitive drum 101 is brought to a developing station, where the toner is adhered to the electrostatic latent image to form a yellow toner image.
The yellow toner image is firstly-transferred onto an intermediate transfer belt 105a of an intermediate transfer device 105. The intermediate transfer belt 105a is wound around and extending between rollers 105c, 105d and 105e and is urged against the photosensitive drum 101 by a first transfer roller 105b to form a first transfer portion T.sub.1 therebetween. The yellow toner image on the photosensitive drum 101 is firstly-transferred onto a surface of the intermediate transfer belt 105a by the first transfer roller 105b at the first transfer portion T.sub.1. After the first-transferring, residual toner remaining on the surface of the photosensitive drum 101 is removed by a cleaning device 106.
Then, the above-mentioned process (charging, exposure, developing, firstly-transferring and cleaning) is repeated regarding the remaining colors, i.e., magenta, cyan and black. In this way, four color toner images are superimposed on the intermediate transfer belt 105a.
At a second transfer portion T.sub.2, these four color toner images are collectively secondary-transferred onto a transfer material P conveyed from a sheet supply cassette 109 through a sheet supply roller 110 in a direction shown by the arrow. After the secondary-transferring, the transfer material P is conveyed, by a convey device 111, to a fixing device 112, when the four color toner images are fixed onto the transfer material with heat and pressure. Then, the transfer material is discharged onto a discharge tray (not shown). After the secondary-transferring, residual toner remaining on the surface of the intermediate transfer belt 105a is removed by a cleaner 108.
On the other hand, Japanese Patent Application Laid-Open No. 4-125676 discloses an image forming apparatus in which a mono-color mode and a multi-color mode can be selected in which, in the multi-color mode, a peripheral speed of an intermediate transfer member is switched from a high speed to a low speed after all color toner images were transferred from a photosensitive drum, thereby transferring the toner images onto a transfer material conveyed at a low speed. After all of the toner images on the intermediate transfer member were transferred, the intermediate transfer member is switched from the low speed to the high speed. Further, after the transferring, the toner images on the transfer material are fixed at a low speed by means of a fixing device.
However, the above-mentioned Japanese Patent Application Laid-Open No. 4-125676 does not disclose a relation between a distance from a position where the toner images are transferred from the photosensitive drum to the intermediate transfer belt to a position where the toner image are transferred from the intermediate transfer belt to the transfer material and a length of the toner image transferred onto the intermediate transfer belt, along a shifting direction of the intermediate transfer belt.
In an image forming apparatus shown in the FIG. 4, if a fixing ability for a full-color image tries to be improved, the entire apparatus will be made bulky.
In the above-mentioned image forming apparatus, a length L.sub.p of a predetermined thick sheet (105 g/cm.sup.2 or more) or an OHT (transparent sheet for an overhead projector) in a conveying direction is selected to be greater than a distance L.sub.T1-T2 from the first transfer portion T.sub.1 to the second transfer portion T.sub.2 along the shifting direction of the intermediate transfer belt 105a so that, before the firstly-transferred is completed, a tip end of the toner image reaches the second transfer portion T.sub.2.
In case of a transfer material P having great heat capacity such as the predetermined thick sheet or the OHT, since the fixing condition thereof differs from the fixing condition of the normal transfer material P (105 g/cm.sup.2 or less), after the full-color image was transferred onto the transfer material P, the fixing speed of the transfer material P is decreased, thereby compensating insufficient heat amount. Thus, in the image forming apparatus using the intermediate transfer belt 105a, after a trail end of the transfer material P passes through the second transfer portion T.sub.2, the speed of the transfer material is reduced to lengthen the fixing time period, thereby obtaining the optimum fixing ability. Accordingly, a distance L.sub.T2-FIX between the second transfer portion T.sub.2 and a fixing portion FIX of the fixing device 112 is set to be greater than the length L.sub.p of the transfer material P in the conveying direction thereof, with the result that a bulky convey device 111 must be arranged between the second transfer portion and the fixing portion, thereby making the entire apparatus bulky.
Further, after the firstly-transferring of the toner image from the photosensitive drum 101 to the intermediate transfer belt 105a at the first transfer portion T.sub.1 is completed, before the secondary-transferring of the toner images from the intermediate transfer belt 105a to the transfer material P at the second transfer portion T.sub.2, the intermediate transfer belt 105a is rotated by one revolution; meanwhile, by reducing the speed of the intermediate transfer belt 105a, the poor fixing can be avoided and the entire apparatus can be prevented from becoming bulky. However, when the intermediate transfer belt 105a is idly rotated by one revolution, the through-out of the mage formation is worsened.
The above problems similarly occur when an intermediate transfer drum is used as the intermediate transfer member.